Certain undercarriages include a steerable bottom portion, with this applying in particular to nosewheel undercarriages, thereby enabling the aircraft to be steered on the ground. For this purpose, the undercarriage is fitted with actuators (jacks in a push-pull configuration, a rack engaging a pinion) enabling the steerable portion to be pivoted in response to a steering order, e.g. generated by means of a steering wheel in the cockpit.
Feedback is provided by means of an angular position sensor arranged on the undercarriage to measure the angular position of the steerable portion and to deliver an angular position signal that is used for forming a feedback loop for servo-controlling angular position.
In known manner, the angular position sensor is duplicated by means of a second angular position sensor so that two angular position signals are generated. Only one of these signals is used for servo-controlling angular position, with the other angular position signal being used for monitoring proper operation of the first angular position sensor. In the event of disagreement between the two signals, servo-control is interrupted and the steerable portion of the undercarriage is allowed to swivel freely. The aircraft pilot can then steer the aircraft by differential braking.
It has been found that under certain circumstances, in particular when making tight turns, or indeed when traveling in a straight line while braking, or merely in the event of the aircraft being nose-heavy, thereby giving rise to a large static force on the undercarriage, that the signals delivered by the angular position sensors may be offset by a certain amount relative to the steering angle actually achieved by the steerable portion of the undercarriage.